I am trying to monitor external service (which is exporter of cassandra metrics) in prometheus-operator. I installed prometheus-operator using helm 2.11.0. I installed it using this yaml:

apiVersion: v1
kind: ServiceAccount
metadata:
  name: tiller
  namespace: kube-system
---
apiVersion: rbac.authorization.k8s.io/v1
kind: ClusterRoleBinding
metadata:
  name: tiller
roleRef:
  apiGroup: rbac.authorization.k8s.io
  kind: ClusterRole
  name: cluster-admin
subjects:
  - kind: ServiceAccount
    name: tiller
    namespace: kube-system

and these commands on my kubernetes cluster:

kubectl create -f rbac-config.yml
helm init --service-account tiller --history-max 200
helm install stable/prometheus-operator --name prometheus-operator --namespace monitoring

Next, basing on article: how monitor to an external service

I tried to do steps described in it. As suggested I created Endpoints, Service and ServiceMonitor with label for existing Prometheus. Here are my yaml files:

apiVersion: v1
kind: Endpoints
metadata:
  name: cassandra-metrics80
  labels:
    app: cassandra-metrics80
subsets:
- addresses:
  - ip: 10.150.1.80
  ports:
  - name: web
    port: 7070
    protocol: TCP

apiVersion: v1
kind: Service
metadata:
  name: cassandra-metrics80
  namespace: monitoring
  labels:
    app: cassandra-metrics80
    release: prometheus-operator
spec:
  externalName: 10.150.1.80
  ports:
  - name: web
    port: 7070
    protocol: TCP
    targetPort: 7070
  type: ExternalName

apiVersion: monitoring.coreos.com/v1
kind: ServiceMonitor
metadata:
  name: cassandra-metrics80
  labels:
    app: cassandra-metrics80
    release: prometheus-operator
spec:
  selector:
    matchLabels:
      app: cassandra-metrics80
      release: prometheus-operator
    namespaceSelector:
      matchNames:
      - monitoring
  endpoints:
  - port: web
    interval: 10s
    honorLabels: true

And in prometheus service discovery page I can see:

That this service is not active and all labels are dropped. I did a numerous things trying to fix this, like setting targetLabels. Trying to relabel the once that are discovered, as described here: prometheus relabeling But unfortunately nothing works. What could be the issue or how can I investigate it better?